The production of .beta.-carotene by the fungus Phycomyces blakesleeanus depends on the culture media and conditions but it is generally small. In the dark, this fungus produces about 50 micrograms of the pigment per gram of dry material (.mu.g/g dry weight), an amount inappropriate for the desired object.
Various ways of increasing this production are known. One of them consists in the addition of vitamin A to the normal culture medium of this organism. This addition causes the accumulation of up to 2,000 .mu.g/g dry weight of .beta.-carotene, but the required concentrations of the vitamin are prohibitively high. Amounts up to 2,000 .mu.g/g dry weight have been observed under the best conditions in the presence of .beta.-ionone.
Another way of increasing the production of .beta.-carotene already described, is the formation of the strains containing nuclei of the two known wild sexual types of Phycomyces, which are represented by the symbols (+) and (-). The method for obtaining this type of strain, denominated intersexual heterokaryon, has been described. These strains accumulate up to 500 .mu.g/g dry weight of .beta.-carotene, they have a peculiar morphology, with formation of small hyphae areas, or pseudophores, and they are unstable: they tend to segregate the components homokaryotically.
In the Mucorales, the interaction between the mycelia of opposite sex leads to a greater carotenogenes is through the formation of trisporic acids. The cultures mixed with strains of Blakeslea trispora of opposite sex in the presence of .beta.-ionone, have been considered as promising for the production of .beta.-carotene. However, it is difficult to maintain the appropriate sex relations in large cultures.
It is also known that the production of .beta.-carotene of the wild type is controlled by the gene carS and that strains genetically altered by mutation in said gene produce up to 4000 .mu.g/g dry weight of .beta.-carotene. Two of the known strains of this type are the strain C115 obtained from a wild strain having a (-) sex, and the strain M1 obtained from a wild strain having a (+) sex. The carS mutants are still sensitive to vitamin A, but a double mutant, strain S106, has been obtained which reaches 6000 .mu.g/g dry weight. The new mutation, car-102 makes the S106 insensitive to vitamin A. The stimulating canal activated by vitamin A has thus been converted as constitutive of this strain.
It is known that the three mentioned types of stimulations, presence of vitamin A, position of intersexuality, and alteration of the gene carS, act independently from one another.